This invention relates to signal processing apparatus and methods for print scanners. More particularly, it relates to the adaptation of such apparatus and methods to the control of printing apparatus such as web printers, for example for evaluation and validation of Universal Product Code (UPC) symbols.
Recently, the grocery and food processing industries have developed a universal product code whereby each product is assigned a unique designation, in the form of a series of lines and spaces which decode to a multiple digit representation. Each UPC symbol is designed to be unobtrusively printed on the labels of even small items, and each therefore involves a rather high degree of graphic precision. In common methods of use, the supermarket check-out or the like is provided with either a light pen or laser scanner, which is contacted with the symbol on the goods; the symbol thereby is automatically decoded for check-out and inventory purposes.
Use of the UPC symbols has added considerable difficulties for the graphic arts industry. In particular, the necessity of printing accurate, valid UPC symbols has tended to limit the production rate of the printers which generate the labels and packaging stock on which the symbol is printed. UPC related problems have been most acute in two types of printing, specifically those involving an expensive print stock and those in which the stock is crude and therefore not particularly amenable to high precision printing. As an example of the high cost stock, metallic cans for soda, beer, or the like typically cost more than the contents thereof. Hence, although that printing is generally fairly precise, even a relatively small run of invalid UPC symbols will be quite expensive. An example of the relatively crude, poor precision class of material is milk carton stock, which is produced at relatively high speeds with a quality on the border-line of unacceptability from the standpoint of valid UPC symbols. In view of the tremendous production rates involved for these materials, poor UPC quality, unless detected speedily, also results in a loss of considerable stock.
The readability of the UPC symbols, on whatever form of stock, is hindered by a variety of factors. First, since the UPC information is encoded in the form of lines and spaces of varying widths, it is necessary to have a rather sharp demarcation between adjacent lines and spaces. Further, with respect to the wider printed lines, it is desirable to have the ink density be consistent from side to side, and to avoid a thinner application of ink at the center of the bar, which at its extremes might be erroneously recognized as a space. Finally, recognition of the symbol may be impaired by the location of spurious ink blots or other imperfections in the space, which might be erroneously sensed as a bar.
Objects of the present invention, in both its method and apparatus aspects, include provision for scanning the printing stock as it is produced, and for providing signal processing capability whereby overall print quality may be determined and controlled.
It is a further object to provide such functions even when an indeterminate, variable amount of ambient light also is introduced in the scanning operation.
More specific objects of the present invention include establishment of threshold levels whereby bars and spaces of the UPC symbol may be distinguished from one another and from printing imperfections as discussed above, and also for development of a print contrast ratio as an index of overall print quality.
On a still more detailed level, it is an object of the present invention to provide recognition of thresholds on a time variant basis, and which accounts for signal evelope curvature introduced by a rotating scan across a flat surface.